Arsenic trioxide has conventionally been produced by oxidation roasting of arsenopyrite or arsenic sulfide. But recently, it has been produced by purifying the recovery from the exhaust gas (flue gas) from the non-ferrous smelting which treats sulfide ores of copper, lead, zinc, etc.
The exhaust gas, that is, flue gas is subjected to dust collection treatment, and the uncollected residual arsenic values are collected in scrubbing water and passed to the waste water treatment plant.
The arsenic values in the scrubbing water is precipitated as arsenic sulfide by the addition of hydrogen sulfide, sodium hydrosulfide, or the like, the precipitate is separated, roasted, sublimed and condensed, and the arsenic is recovered as arsenic trioxide. However, this process requires large-scale equipment, and the operation thereof consumes a large amount of energy. Also, dust prevention measures are required because this is a dry process.
Acid leaching of the above-described arsenic sulfide is also practiced for the recovery of arsenic trioxide. This process is complicated, and therefore is not suitable as a commercial operation. Extraction processes in which an organic solvent, such as TBP, is used has been proposed, but they have not yet been industrially applied.
A process for recovering arsenic trioxide from the scrubbing water for the flue gas by crystallizing arsenic trioxide therein has been proposed (Laid-Open Patent Publication No. 118028/82). This process comprises removing fluorine compounds by adding a neutralizing agent to the scrubbing water for the flue gas, if necessary, and recovering arsenic trioxide crystals by vacuum evaporation or cooling. The defect of this process is that an unnegligible amount of plaster (calcium sulfate) is contained in the crystals, and it is difficult to consistently obtain arsenic trioxide of the commercially required purity of 99%.
We reexamined every step of the above-described prior art processes, and have completed a process for recovering arsenic trioxide having a purity of at least 99% with low plaster content by a simple unit operation with small consumption of energy.